Current three dimensional laser beam triangulation inspection systems use CCD (Charge Coupled Device) linear or rectangular arrays to read the position of a surface to be measured. A position sensing detector (PSD) is an alternative sensing device that offers the benefits of greater sensing element width which is important for scanned beams as well as lower cost and greater speed as a consequence of simpler design and absence of the need to serially clock out a string of data samples. A PSD is an analog device that provides an output current ratio proportional to the position of a light spot falling along its length. The CCD linear or rectangular array provides an output that is an image of light distribution on its surface.
Unfortunately reliable, accurate data is sometimes more difficult to obtain with a PSD than with a CCD because any stray light striking the PSD photo-sensitive surface or surrounding substrate combines with the result generated by the desired measurement light to introduce an error. With CCD's the stray light signals along the length of the array can usually be readily separated from the desired signal since the amplitude distribution of stray light on the CCD array is quite different than that of a focused laser spot. With PSD's, once the undesired light produces a signal, it mixes with the desired signal and can't be removed.